The present invention relates to a method for the simultaneous measurement of spin resonance signals, in particular NMR signals from layers located at different depths in a test volume which is subjected to a homogeneous magnetic field in the Z direction and to successive sequences of pulse-shaped gradient fields as well as to multiple-component rf excitation pulses P.sub.n that are synchronized in time with the said gradient fields and whose n components P.sub.m excite, due to their respective frequency .omega..sub.m, a spin resonance of the Larmor frequency .omega..sub.m in a defined layer m of the test volume, a variation in space of the Larmor frequency .omega..sub.m being effected in the Z direction by application of a layer-selective gradient and a read gradient being applied in a direction perpendicular to the Z direction at least for the time of appearance of the spin echo signal.
Methods of this type have been known before from nuclear spin tomography, in particular in connection with the two or three-dimensional imaging techniques. If it is desired to produce a three-dimensional image of certain physical properties of a sample, such as the spin density or flow rate, using 3DFT techniques, this can be achieved only by measuring successively a great number of successive two-dimensional segments of the test volume, a process which is extremely time-consuming and, consequently, rather unsuitable for recording transient processes of the type typically encountered in flow measurements. Although there have been known 3DFT methods where a plurality of disk-like layers of the test volume are subjected to simultaneous spin excitation through a multiple-component excitation pulse, it is a requirement in this case that the individual test volume layers under examination must succeed each other coherently so that it is not possible to eliminate such spatial areas in the volume observed which are of no interest for the observer.
From U.S. Pat. No. 4,843,322 it has been known that a multiple-component excitation pulse may contain terms which are modulated by a K.sub.y -dependent phase factor that makes the images obtained from the different excited layers distinguishable during reconstruction thereof. However, the publication does not contain concrete rules as to how the different layers are to be measured, nor does it suggest an algorithm for the phase displacement.